1. Field of the Invention
The present invention relates to a sheet finisher, an image forming apparatus using the same, and a sheet finishing method, particularly to a sheet finisher to perform a folding process of a printed sheet, an image forming apparatus using the same, and a sheet finishing method.
2. Description of the Related Art
Hitherto, there is known a sheet finisher which is placed downstream of an image forming apparatus, such as a copier, a printer or an MFP (Multi-Function Peripheral), and performs a post-processing, such as a punching process or a stitching process, on a printed sheet.
Recently, the function of this sheet finisher is diversified, and a sheet finisher is proposed which has, in addition to the function of the punching process and the stitching process, the function of a folding process to fold a part of a sheet, and the function of a saddle-stitching and folding process to staple the center of a sheet and then to fold the sheet at the center (JP-A 2004-59304, JP-A 2003-182928, etc.)
In the sheet finisher having the function of the saddle-stitching and folding process, it becomes possible to form a booklet (to bind a book) from a plurality of printed sheets.
In the saddle-stitching and folding process proposed hitherto, after the canter of sheets is stitched with staples or the like, a process is performed in which a fold line is formed on the stitched part by a pair of rollers called fold rollers and folding is performed. At this time, a plate-like member called a fold blade is brought into contact with the stitched part of the sheet bundle, and is pressed into a nip of the fold roller pair to form the fold line on the sheet bundle.
However, the time in which the folded part of the sheet bundle is pressed by the nip of the fold rollers is short, and the whole folded part is simultaneously pressed by the nip of the fold rollers, and accordingly, the pressure is dispersed to the whole fold line. Thus, the fold line formed by the fold rollers becomes the fold line to which the pressure is not sufficiently applied. Particularly, in the case where the number of sheets is large, or in the case where a thick sheet is contained in the sheet bundle, the fold line often becomes incomplete.
In order to deal with this problem, JP-A 2004-59304 or JP-A 2003-182928 discloses a technique in which a roller called a reinforce roller is additionally provided, and the fold line formed by the fold rollers is reinforced by this reinforce roller.
In the technique disclosed in JP-A 2004-59304, the sheet bundle pushed out from the fold roller is temporarily stopped on a guide plate, and the reinforce roller is moved along the fold line while applying pressure to the fold line of the sheet bundle from above. The fold line nipped between the guide plate and the reinforce roller is reinforced by the pressure generated between the guide plate and the reinforce roller.
JP-A 2003-182928 also discloses a technique in which a fold line pushed out from a fold roller is nipped in a nip of a pair of reinforce rollers, and the pair of reinforce rollers is moved along the fold line to reinforce the fold line.
The reinforce roller is moved in the direction along the line of the fold line (that is, the direction orthogonal to the transport direction of the sheet bundle). Since the reinforce roller is pressed by a spring or the like in the direction toward the guide plate (or the other reinforce roller of the pair of reinforce rollers), it is in direct contact with the guide plate (or the other reinforce roller) in an area where there is no sheet bundle, the sheet bundle is nipped at the end of the sheet bundle, and then, the reinforce roller moves on the fold line. Thus, when the sheet bundle is nipped, there is a case where the end of the sheet bundle is turned up. Besides, there is a case where, when the roller moves along the fold line, a wrinkle occurs in the vicinity of the fold line by the friction between the sheet and the roller.
In order to prevent the end from being turned up or the wrinkle from occurring in the vicinity of the fold line, JP-A 2003-182928 discloses a technique in which a film-like flexible member made of a resin such as polyethylene terephthalate (PET) is made to intervene between the sheet bundle and the reinforce roller.
The occurrence of turning-up and the wrinkle can be certainly prevented by causing the flexible member to intervene between the sheet bundle and the reinforce roller. However, when the flexible member is made to intervene, sliding occurs between the reinforce roller and the flexible member, and there is a possibility that the sufficient pressure is not applied to the fold line of the sheet bundle. Besides, the area of a pressure application point by the reinforce roller is equivalently widened by the interposition of the flexible member, and the pressure to the fold line is reduced.